1. Field of the Invention
The present invention is directed toward a filtering and masking technique for use with an output of a magnetic scanning device such as a scanning SQUID microscope. In particular, the invention is directed toward filtering and masking for converting magnetic field images into images of current density, converting magnetic field images into images of magnetic charge density, and converting magnetic field images into images of magnetization. Additionally, the filters of the present invention can be applied to converting electric field images into electric charge density, converting electric field images into images of voltage, and converting electric field images into images of polarization. The invention is useful for such purposes as obtaining an image of currents flowing in a microelectronic circuit, imaging of magnetic media such as computer disk drives, films of magnetic materials, and also for imaging of other integrated circuit applications. In particular, the invention is directed toward a method of using restrictions placed upon circuit geometry to improve the spatial resolution of an image of currents flowing in an electronic circuit. The present invention can also be used with the output of an electric field scanning device such as a scanning SET (Single Electron Transistor) microscope to obtain an image of the voltage level on wires or electrical components in the circuit.
2. Description of the Related Art
A number of techniques have been developed to image magnetic fields at length scales of a few xcexcm or smaller. These include decoration techniques, magnetoresistive or Hall probe sensors, flux-gate magnetometers, magneto-optic thin films, magnetic force microscopy, and electron beam interferometry. In particular, it has been determined that a magnetic flux microscope using a thin-film Superconducting Quantum Interference Device (SQUID) as a scanning device, such as that disclosed in U.S. Pat. No. 5,491,411, which is hereby incorporated by reference, can be used to obtain a magnetic image with a sufficiently high spatial and field resolution. Another example of a suitable scanning device is disclosed in U.S. Pat. No. 5,894,220, which is also incorporated by reference.
In the field of semiconductor/microelectronics testing, there is a need to measure the current flow and image the related current paths in semiconductor circuits and microelectronic devices. The devices currently used have proved to be of limited use in these endeavors because the images created are unclear due to noise, and allow for limited spatial resolution. Therefore, they cannot effectively image the flow of small currents in microelectronic structures.
It has also been determined that an SET microscope can be used as a scanning device, for scanning electric fields and imaging voltage levels on wires, rather than magnetic fields and imaging currents in wires using a scanning SQUID.
With the above discussed limitations of the prior art in mind, it is an object of the present invention to provide a spatial frequency filtering and masking technique, hereinafter referred to as spatial filtering and masking, or simply xe2x80x9cfiltering and maskingxe2x80x9d, that can be used in conjunction with an output of a magnetic scanning device such as a scanning SQUID microscope to convert images of magnetic fields into images of current paths or wires in the microelectronic circuits. Such images enable identification of circuit faults, as well as identifying circuit structures without physically dismantling the circuit or the device containing the circuit. In particular, it is an object of the present invention to use novel signal processing filters to increase the signal-to-noise ratio of images of current flow or wires in microelectronic circuits.
It is also an object of the present invention to increase the spatial resolution obtainable from magnetic field images of microelectronic circuits when the images are mathematically transformed into images of current density flowing in a circuit.
It is yet another object of the invention to improve magnetic imaging techniques, so as to better enable them to locate the position of short circuits in microelectronic circuits.
In accordance with the above discussed objects, the present invention provides a spatial filtering and masking technique that cleans up background noise in Fourier space, and eliminates the edge effect from a finite sized data set. Without the spatial filtering and masking technique of the present invention, it is very difficult to obtain a high resolution picture of current paths because the background noise is exponentially amplified. In other words, the signal to noise ratio is small without the spatial filtering and masking technique. The spatial filtering and masking technique of the present invention can be used together with a scanning SQUID microscope, or other magnetic scanning device to convert magnetic fields into images of current paths or wires in the microelectronic circuits so that faults (if they exist) can be located. The filtering and masking technique can be incorporated into software which operates on a general purpose or special purpose computer, and which performs a Fourier transform of a magnetic field and converts the data into current paths.
Another object of the present invention is to provide a spatial frequency filtering and masking technique that can be used in conjunction with an output of an electric field scanning device such as a scanning SET microscope to convert electric fields into images of voltages on wires in microelectronic circuits. Such images enable identification of circuit faults, as well as identifying circuit structures without physically dismantling the circuit or the device containing the circuit. In particular, the present invention utilizes novel signal processing filters to increase the signal-to-noise ratio of images of voltage level on wires in microelectronic circuits.
It is also an object of the present invention to increase the spatial resolution obtainable from electric field images of microelectronic circuits when the images are mathematically transformed into images of voltage levels in a circuit.